1. Field of the Invention
The present invention generally relates to semiconductor thermoelectric elements and more specifically, to a nanoporous semiconductor material and fabrication technique for use as thermoelectric elements.
2. Description of Prior Art
The basic component of the modern thermoelectric device utilizes semiconductors in what is called the Peltier Couple. The Peltier Couple essentially consists of metallic conductors (which ideally exhibit negligible thermoelectric effects) which are coupled through a doped n-type and p-type semiconductor. The nature of the semiconductors allow for a larger energy to be released or required due to changes in transport energy as electrons move between metal to semiconductor and back again. The usefulness of a particular material resides for the most part on three material properties: Seebeck coefficient S, electrical resistivity .rho., and thermal conductivity k.
Ideally, thermoelectric materials must exhibit high electrical conductivity and low thermal conductivity. The usefulness of any thermoelectric material can be described by a figure-of-merit (Z) expressed as: EQU Z=.alpha..sup.2 .sigma./.kappa.
where .alpha. is the Seebeck coefficient, .sigma. is the electrical conductivity, and .kappa. is the total thermal conductivity, a sum of lattice and electronic components. The .alpha. term decreases with increases of the free-carrier concentration, which is the opposite of electrical conductivity. The optimum free-carrier concentration to maximize Z in bulk thermoelectric material is such that the material becomes degenerately doped, which is about 10.sup.19 cm.sup.-3. Semiconductor materials (with band gaps that are optimized for operating temperature) are considered ideal thermoelectric materials compared to metals or insulators.
Multiple stage thermoelectric devices (consisting of mating couples to form individual stages) are designed where energy from one stage is used as input to the next stage so that a larger temperature difference is attainable. No viable device has yet been developed since there awaits in the prior art the development of thermoelectric materials with higher FOMs than are presently available. An example of this non-viability is shown in discussing thermoelectric for cryogenic coolers used in thermal image detectors.
A thermal image detector of either the cooled or uncooled type is required to be isolated from random noise and have been proposed by structural modifications of the material structure. Experimental thermal isolation support structures have been proposed for uncooled thermal detectors utilizing aerogels, which are oxide containing sol-gels derived materials, supercritically dried with porosities up to 98%. A cooled detector is required to be at very low operating temperatures so as to detect minute background temperature variations, whereby the low temperature performs the "isolate" type function. Experimental thermoelectric materials have been attempted for use in thermoelectric cryogenic coolers of cooled detectors which are made of n and p-type Bi.sub.2 Te.sub.3 material with the appropriate dopant. At present, the best FOM values for semiconductivity are slightly greater than 3.times.10.sup.-3 /.degree.K. In order to reach cryogenic temperatures (120.degree. K. or lower) with a coefficient-of performance of at least 1%, a FOM of 10.sup.-2 /.degree.K. for both n and p legs must be found.
While the prior art has reported using semiconductor thermoelectric elements none have established a basis for a specific material and fabrication technique that is dedicated to the task of resolving the particular problem at hand.
What is needed in this instance is a radically new approach to structural modification of thermoelectric materials and their technique of fabrication so as to produce higher FOMs to make thermoelectric devices practical.